This invention relates to a method and apparatus for forming metal pipes and tubes, especially welded pipes and tubes the seam line of which extends parallel to the longitudinal axis thereof.
In the manufacture of such welded pipes and tubes as electric-welded pipes and tubes the seam line of which extends parallel to the longitudinal axis thereof, a skelp or a strip preslit to a required size is shaped by forming rolls into an almost closed circle, then both edges thereof, facing each other, are welded together. Conventionally, the following methods have been known for preforming the skelp for such welded pipes and tubes.
The first method is known as step roll forming. This method performs stepwise forming operations on a skelp longitudinally threaded through several pairs of horizontal rolls and vertical side rolls disposed along the pass line. In the initial stage, the entire flat skelp, slit to a desired size, is bent by, for example, three pairs of curved horizontal rolls. The degree of bending is not necessarily fixed. The flat skelp is shaped into a U-form instep. In the intermediate stage, the U-shaped skelp is shaped into an O-form, further bending the edges and center thereof by, for example, three pairs of vertical side rolls and one pair of curved horizontal rolls disposed at suitable intervals. By the initial and intermediate stages, the skelp is shaped very close to the desired finished pipe or tube. In the final stage, finned-rolls, each having a circumferential fin, coin the opposite skelp edges to make them ready for welding.
As described above, the step roll forming method forms a pre-slit sklep into a circular form. The shape of the skelp changes rather greatly at each pair of forming rolls, because not many pairs of forming rolls are disposed at intervals along the pass line. This results in springback and stepped edges of the formed skelp. Because of this arrangement which carries out heavy fabrication at each pair of forming rolls, the edges of the skelp become longitudinally elongated at the entrance of each stand, then longitudinally compressed as it draws close to the rolls, and further compressed on leaving the rolls. In the finish-forming stage where individual forming rolls exert great restraining forces, compressive force works on the edges elongated in the initial and intermediate stages. When the skelp is thick enough, this compressive force is absorbed as compressive strain. But thinner skelps are likely to develop longitudinal buckling.
A second method is known as cage roll forming or natural function forming. According to this method, a skelp is preformed into a simple arched shape by a pair of curved horizontal rolls, then given an edge-forming pass through a pair of edge-forming rolls. Following such initial preforming and edge-forming, the skelp passes through a number of small idle rolls or cage rolls disposed along the pass line, whereby the skelp is progressively and smoothly pressed inward from the outside of an O-form. The skelp formed by this method has hardly any vertical bend, springback, edge elongation or buckling. A tube mill according to U.S. Pat. No. 3,472,053 is an example of equipment to carry out the forming operation of this type.
But this cage roll forming method forms the skelp edges to only minor curvatures in the initial and intermediate stages, as compared with the final curvature of the finished pipe or tube. The entire width of the skelp also is bent to a limited degree. The greater part of edge-forming is accomplished by the finned-rolls in the finish-forming stage. Accordingly, this finish-forming calls for a large forming load and, therefore, a very rigid finish-forming apparatus. If the forming load and apparatus rigidity are low, it is difficult to form a square groove, necessary for performing satisfactory butt welding, between the edges of the rounded skelp.